DE69428287T2 - METER - Google Patents

Description

The present invention relates to a measuring device for measuring the amount of solids contained in a liquid.

In many processes there is a need to know the amount of solids contained in a liquid. In some cases very small quantities of particles contained in a liquid, i.e. concentrations of a few ppm, must be measured, such as in waterworks to control drinking water purity. In other cases the concentration of fibers suspended in water used in paper mills must be measured to control the various stages during the papermaking process. In other industries, such as the food industry, there is an interest in measuring the moisture content of food products.

It is previously known to use photometric measuring devices using pulsed light, mostly infrared (IR) light, to measure the concentration of particles in liquids. Swedish Patent Nos. SE-B-382 166, SE-B-453 015 and European Patent No. EP-A-96 696, for example, disclose such devices. The disadvantages of the earlier devices, however, are numerous.

When a beam of light passes through a material, the optical radiation is attenuated according to the following formula:

P = P₀e-u(λ)I

where P₀ is the initial power of a beam, P is the corresponding power when the beam has travelled a distance 1 or, when used for concentration measurement, the concentration of particles and u(λ) is the attenuation coefficient. The attenuation coefficient is composed of two components: the absorption coefficient λ and the scattering coefficient s, so that u = λ + s. Of these two components, λ is very dependent on the wavelength. Thus, the choice of wavelength is important and therefore infrared light is chosen for which the absorption is low.

This is important according to the known transmission techniques, since the light transmitted through a suspension is measured and the differences between the emitted and detected light are a measure of the concentration of solids in the liquid. Too much light must not be absorbed, which leads to low measurement signals.

However, problems with the known technology remain, since the detected transmitted energy depends exponentially on the concentration according to the above formula, so that the detected energy is significantly reduced at higher concentrations of solids. In order to achieve reasonable measured value signals, it is necessary to use high energies and to transmit short pulses at long intervals in order not to overload the IR diode.

The exponential relationship also presents other problems, such as a need for post-processing of the acquired signals to obtain signals that vary according to a linear relationship.

The transmission technology and the exponentially decreasing energy values that are detected in relation to the concentration of solids suggest that the measuring device often cannot be used directly in the process, as the distance between the transmitter/detector and/or the concentration would be so large that no light could be transmitted. This is solved, as described in Swedish Patent No. 453,015, by diverting part of the flow and forcing it through a pipe with a defined diameter and with the transmitter/detector arranged opposite each other to provide a "two-dimensional measurement". This requires complex piping constructions to divert part of the flow to be measured.

Disadvantages in the strength of signals that are not related to concentration, such as reflection disturbances, temperature dependence of the IR diodes in the exchange of light, deposits on the sensors and so on, ie noise, are difficult to compensate with the known two-dimensional transmission technique. In Swedish Patent No. 453,015 a reference device is disclosed which consists of a measuring reference unit in thermally conductive contact with a liquid supply tube, the unit being designed to compensate for the temperature dependence of the IR diodes and the remaining components in the system as well as incident scattered light. These reference units do not measure through the liquid but along an obstruction-free path within the enclosure of the device.

Patent No. EP-A-96 696 describes a device for directly measuring the moisture content in a specific material by measuring the reflection of IR radiation from the material, instead of using transmission measurement to detect reflections. Interference filters are placed in front of the sensors to separate the relevant wavelengths to be analyzed and which depend on the material to be measured. Also disclosed in this patent is an adjustment device to compensate for the temperature dependence of the IR diodes.

WO-A-82/03688 discloses a particular wavelength light reflectance measuring device for measuring the brightness of paper pulp, including first and second fiber optic bundles; a plurality of light sources and a light sensor.

WO-A-93/15389 discloses a method and apparatus for determining refiner pulp properties by measuring the change in the radiation spectrum caused by the pulp at a minimum of four wavelength bands: The obvious disadvantages of the conventional technologies lead to limitations in the measuring range, so that prior art measuring devices only measure within a relatively narrow measuring range for which they are specifically designed. Despite the availability of sophisticated electronics, the need for better light emitters and receivers and reference devices continues to be a fundamental problem when using the conventional technologies.

The present invention aims to utilize the advantages of pulsed light when measuring the amount of solid concentration in liquids while eliminating the existing disadvantages of the prior art.

According to the invention, there is thus provided a device for measuring the amount of solids contained in a liquid with light, which also includes infrared and ultraviolet, the device being equipped with at least two pairs of light transmitting devices and at least one detection device, characterized in that the at least two pairs of light transmitting devices are arranged in a cross so as to obtain a three-dimensional illumination of particles of the solids, and in that the transmitting devices in the respective pairs of transmitting devices are arranged to transmit pulses of coherent light in pairs simultaneously, i.e. light pulses having the same wavelength, while the wavelengths of the pairs of transmitting devices are different from each other.

When an irradiance, i.e. the radiation from a light source that includes infrared and ultraviolet, strikes a solid contained in a liquid, some energy is absorbed. The amount of energy absorbed is determined by the absorption coefficient and is dependent on wavelength as described above. This absorption provides a spectral shift toward longer wavelengths on the spectral scale, so that the wavelengths with higher frequencies are shifted more on the spectral scale than wavelengths with lower frequencies. The radiance reflected from a solid that is detected by the detector is spectrally shifted compared to the incident radiance.

When two or more pulses of light are emitted at different selective wavelengths, spectral shifts are achieved and the difference between these provides what is called spectral contrast. The proportional change caused by the spectral shift can be used to obtain a measure of concentration. The amount of shift is different for different wavelengths and the contrast is a direct function of mass. If the mass is changed, the contrast will change in direct proportion to the change in mass.

For example, a sensor is provided with four transmission devices, one on each arm of a cross. The transmission devices are at the same distance from the center of the Cross and preferably with the same inclination. Oppositely arranged transmitters are connected to each other so that they simultaneously transmit a light pulse in the form of a beam of radiation having a specific wavelength for each of the pairs of transmitters (in this disclosure referred to as λx and λy respectively), one of the wavelengths being used as a reference. The particles are thus illuminated with two wavelengths and with a certain angle of rotation with respect to each other. Subsequent pulses are emitted by the two pairs at very short time intervals. A "three-dimensional" illumination of the particles is achieved by this cross (here referred to as "cross"). However, the sensor can be provided with several pairs of oppositely arranged transmitters transmitting at different wavelengths in order to obtain more information about the medium to be measured, such as the amount of particles below a certain size, etc. One of the wavelengths used forms a reference as described above. However, the configuration of the cross is maintained so that the transmission devices are arranged on additional arms of the cross at the same distance from the center of the cross and preferably at the same angle.

A detection device is preferably arranged in the center of the cross to detect the light reflected from the particles at two or more wavelengths.

The contrast thus generated due to the spectral difference between the typical wavelengths selected forms an output signal which is a function of irradiance and the pulse ratio (contrast) λx - λy (or transmitter value minus reference value). The measured value is the sum/integral of the total contrast detected by the detector. This measured value contains the spectral difference of the two wavelengths which includes irrelevant signals, i.e. noise.

However, since the noise is associated with the two impulse responses and is independent of the wavelengths, i.e. the noise is the same for the two impulse responses, this noise can be easily removed by the means of a differentiator and the remaining part is the spectral contrast which is a direct function of the mass. The measuring device therefore does not require any additional reference or compensation devices. The detector signal is also linear after filtering.

An important advantage of the proposed method is that the output signal detected by the detection device is already digitized at the measuring point, which leads to technical advantages from the measurement point of view.

This measuring method provides much more reliable measurements, since all irrelevant signals and interference can be easily filtered out, but in particular the measuring range can be significantly extended and is in principle only limited by whether the measuring sensor can be inserted into the medium to be measured.

The measuring device according to the present invention is described in detail below and in combination with a preferred embodiment shown in the accompanying drawings, wherein.

Fig. 1 is a perspective view of a sensor according to the preferred embodiment of the present invention;

Fig. 2 shows the so-called cross with two pairs of sensors and the so-called radiation lobes generated by the light pulses from the transmitters;

Fig. 3 shows the system of the measuring device and the pulse generator and the decoder/differentiator synchronized by the pulse generator;

Fig. 4 shows the origin of the spectral contrast during the transmission of two different wavelengths;

Fig. 5 shows the time relationship between the pulses and the responses to the transmitted pulses.

A measuring device of a preferred embodiment of the present invention, as shown in Fig. 1, includes a probe 1 having a tubular protective enclosure 2. A refractor 3 is attached to one end of the protective enclosure 2. The refractor is made of, for example, PTFE. A body 4 made of glass, PTFE or any other light-transmitting material is attached to the refractor 3. The body may have a pointed, rounded, aspherical or teardrop shape so as not to cause turbulence around the end of the sensor and to reduce its flow resistance. The refractive index between body 4 and water is almost zero so that very little light is reflected from body 4 and generally particles directly in front of sensor 1 are illuminated.

In the preferred embodiment, five optical fibers are used, four for the transmission devices (Sx, Sy) and one for the detection device (D₀) drawn in parallel and enclosed in the protective enclosure 1. The optical fibers are drawn through enclosure 2 and terminate in a layer formed between refractor 3 and body 4. Since the transmission devices or their optical fibers form a unit, they have the same reference numerals in the figures for simplicity. As can be seen in Fig. 1-2, the conductors for the transmission devices (Sx, Sy) and detection device D₀ are arranged with respect to each other so that they form a cross, as can be seen from below in sensor 1, which has the conductor for sensor (D₀) arranged at the intersection of the cross.

The opposite ends of transmitters (Sx, Sy) are connected to four light-emitting diodes. The light-emitting diodes are selected in such a way that the diodes connected to Sx, Sy transmitters emit at the same wavelength at the same time. The wavelengths are selected to be within the wavelengths of infrared and ultraviolet light, i.e. the light-emitting diodes emit light in pairs in two "planes". For example, the light-emitting diodes connected to Sx may operate at a specific wavelength that is different from the light-emitting diodes connected to Sy transmitter and rotated 90 degrees with respect to them. However, other angular relationships may also be used. Fig. 2 shows how the pulses from each of the pairs form a beam of radiation to illuminate the particles located in front of sensor 1. The light emitting diodes are connected to a pulse generator of a known type and triggered so that the two pairs of diodes alternately emit pulses of light at short intervals.

The operation is as follows. A clock frequency at which all the electronics of the measuring device operate triggers diodes Sx via the pulse generator so that the diodes emit a light pulse via the two optical fibers Sx of the order of 50-100 us at a wavelength λX. The light pulses illuminate the solids in the liquid arranged in front of the sensor at this wavelength. A very short moment later, the diodes Sy are triggered so that they emit a light pulse at a wavelength λy (≠λx) via the two optical fibers Sy so that the solids arranged in front of the sensor are irradiated at this wavelength. The particles in the liquid are therefore irradiated with short successive light pulses at two different wavelengths and preferably rotated by 90 degrees, a type of cross modulation.

Fig. 4 shows the spectral contrast that therefore occurs due to the spectral difference of the two typically chosen wavelengths when the wavelength λx with a higher frequency is shifted more on the spectral scale than λy with a lower frequency. The sum of the spectral contrast is the difference of the shift of the two wavelengths, which can be described as follows, such that λz = Δλx - Δλy.

Detection device D�0, preferably located in the middle of the λ-cross, registers this difference or contrast. The measured value is the sum of the total contrast detected by the detection device. The spectral difference of the two wavelengths, which includes irrelevant signals, i.e. noise, is included in this measured value. However, since the noise is associated with both impulse responses and this is independent of the wavelengths, i.e. the same for both of the impulse responses, the noise can be removed from the signal of the detection device by the arrangement shown in Fig. 3, which represents a detection device and a differentiator of a known type in synchronization with radiating pulses, so that only the spectral contrast remains. This remaining spectral contrast is proportional to the mass of the solids contained in the liquid being measured. The samples of signals are obtained in this way by the sequences of incident pulses of spectral contrasts.

Fig. 5 shows a time dependence of pulses in a graphical representation for the two pairs of transmitters and the responses from the detector as a result of the light reflected from the solids in the liquid. Some compensation of luminescence and saturation can be achieved by extending one of the light pulses in time and correspondingly shortening the other. The measurement system of the present invention is not limited to only two pairs of transmitters and two wavelengths, but can be extended with multiple pairs of emitters transmitting at specific wavelengths that different from the wavelengths transmitted by other transmitters. For example, the amount of solids smaller than a certain size can be measured by analyzing the spectral contrasts. The measuring system also has the advantage of covering a wide measuring range from a few parts per million to measuring moisture content with the same device, which essentially extends the measuring and application areas compared to the prior art devices, which all operate within a specific measuring range and for a specific application. The measuring principle also has the advantage that the signal acquired by the acquisition device is immediately digital, which provides great metrological advantages, and that the output signal after filtering is linear.

It must be understood from the above that the measuring system described is not limited to liquids, but could also be applied to other media.

It is clear that the invention should not be considered as limited by the above description and the preferred embodiment shown in the drawings, but may be subject to various modifications within the scope defined in the appended claims.

Reference symbols of the drawings

I λ-cross

II Receiver

III Radiation lobes

IV Infrared

V Infrared modulator

VI Measuring cell

VII -Object, -Contrast, -Noise

VIII Photodetector

IX Amplifier

X System synchronization

XI Detection device

XII S/R data demodulator

XIII digital filter

XIV Differentiator

XV relative spectral emission with respect to wavelength

XVI Wavelength

XVII Sensor phase

XVIII Reference phase

XIX Absorption

XX loss

XXI Radiation

XXII. Absorption

XXIII Modulator

Claims (12)

1. Device for measuring the amount of solids contained in a liquid, with light which also includes infrared and ultraviolet, the device being equipped with at least two pairs of light transmitting devices (Sx, Sy) and at least one detecting device (D₀), characterized in that the at least two pairs of light transmitting devices (Sx, Sy) are arranged crosswise; so that a three-dimensional illumination of particles of the solids is obtained and:that the transmitting devices in the respective pairs of transmitting devices are arranged to transmit pulses of coherent light in pairs, i.e. light pulses having the same wavelength (λx, λy) simultaneously, while the wavelengths of the transmitting device pairs differ from each other. 2. Device according to claim 1, characterized by means for successively transmitting light pulses from the pairs of transmitters at short time intervals. 3. Device according to one of claims 1 or 2, characterized in that each of the transmission devices (Sx, Sy) comprises a light source adapted to emit coherent light in pulses and an optical fiber. 4. Device according to claim 3, characterized in that the straight line connecting the ends of the transmission device conductors in one transmission device pair (Sx) and the straight line connecting the ends of the transmission device connection conductors in at least one other transmission device pair (Sy) form a cross (I) in which the lines intersect at a defined angle. 5. Device according to claim 4, characterized in that the angles at which the lines in the cross (I) intersect are equal. 6. Device according to one of claims 1-5, characterized in that the detection device (D₀) is arranged in the intersection point of the cross (I). 7. Device according to one of claims 1-5, characterized in that the conductive fibers (Sx, Sy) run parallel to each other within at least one end region (3) through a sensor device (1) intended to be arranged in the liquid in which the amount of solids is to be measured. 8. Device according to claim 7, characterized in that the optical fibers belonging to the respective transmission device (Sx, Sy) form planes within the region of the end region, the planes separating at a well-defined angle, the planes coinciding with the cross (I). 9. Device according to one of claims 7-8, characterized in that the region of the sensor device (1) consists of a light refraction device (3). 10. Device according to one of claims 3-9, characterized in that the ends of the optical waveguide fibers (Sx, Sy) in the sensor device (1) form a plane and that the ends are covered by a light-permeable body (4). 11. Device according to one of claims 7-10, characterized in that the detection device (D₀) also comprises an optical fiber and that the end of the optical fiber lies in the same plane in the sensor device (1) as the ends of the transmission device conductors (Sx, Sy). 12. Device according to claim 10, characterized in that the body (4) is pointed, rounded, aspherical or teardrop-shaped in order to reduce the flow resistance of the sensor device (1).